SCEC Award Number 12170 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title A High Resolution Lake Cahuilla Chronology to Constrain Earthquakes on the Southern San Andreas System
Investigator(s)
Name Organization
Ray Weldon University of Oregon Thomas Rockwell San Diego State University
Other Participants One graduate student from the University of Oregon
One graduate student from San Diego State University
Professor Ilya Bindeman, UO Stabel Isotope Lab
SCEC Priorities 2a, 1a, 4a SCEC Groups Geology, SoSAFE, USR
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Our work focused on developing a higher-resolution chronology of Lake Cahuilla in which to place the rupture history of several major elements of the southern San Andreas fault system. Towards that end, our work focused on two fundamentally different aspects: 1) correlation of lake strata using oxygen and carbon isotopic ratios to test whether this method could be used to uniquely correlate the many sites with shells to those radiometrically dated; and 2) direct dating of dead stumps that grew below the shoreline of Lake Cahuilla and required the absence of water at those times.
Preliminary oxygen and carbon isotopic analyses shows that lakes have distinctive d18O/d13C values that appear to evolve through the life time of a lake, and that different species record different d13C values at a single time in a lake.
We found many stumps that were inundated by the last highstand of Lake Cahuilla. Dating the outer and inner portions of the wood yields ages that, along with the historical record, indicate that the stumps grew sometime in the interval between AD 1705 and 1720. Combined with the historical record, tree ring records from the Colorado Plateau, and calculations that require lake desiccation to be mostly complete by 1774, the most recent lake must represent a fairly brief filling around AD 1717-1726. As the most recent large rupture of the southern San Andreas fault occurred during the last lake, and these new dates move the timing of this earthquake forward by several decades into the 18th century.
Intellectual Merit New data presented in this report, in conjunction with previous radiocarbon data, the historical record, and rainfall records point to only a single short time interval that may correspond to the final filling of Lake Cahuilla. A brief lake is consistent with shoreline exposures at the northern Superstition Mountain site (Gurrola and Rockwell, 1996), where Lake A produced a notch into the previously deposited shoreline berm. A 1724 date also barely allows for desiccation of most or all of the lake prior to Anza’s visits. Consequently, we take this brief period in the early 1720’s as the timing of Lake A.
Several studies place the timing of the most recent large southern San Andreas fault rupture in Lake A, the youngest full filling. If correct, and if our dates for Lake A hold up with future work, this implies that the lapse time or open interval for the southern San Andreas fault may be as much as 40 years less than currently assessed in UCERF3.
Broader Impacts This grant funded training and lab experience for students from two universities:
Training of graduate students, Erik Haaker, SDSU, and Ashley Streig, UO.
Undergraduate Nick Weldon analyzed oxygen and carbon samples at the University of Oregon.
Exemplary Figure Figure 3. Plot of rainfall patterns interpreted from tree ring records for the Colorado Plateau region since about AD 1300 (Salzerand Kipfmueller, 2005). Here, we plot the dates when various explorers noted water from the Colorado River flowing to the Gulf of California (indicated as RTS – river to south).
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